Polyarylene sulfide resins (hereinafter abbreviated to “PAS resins”) typified by polyphenylene sulfide resins (hereinafter abbreviated to “PPS resins”) have good heat resistance and chemical resistance, and are widely used for applications such as electric/electronic components, automobile parts, water heater components, fibers, and films. In these applications, recently, high-molecular-weight PAS resins have been particularly widely used in terms of high strength and good moldability. However, a high-molecular-weight PAS resin is usually produced by synthesizing a low-molecular-weight PAS resin, and then subjecting the PAS resin to thermal oxidative cross-linking so as to increase the molecular weight. Accordingly, the resulting high-molecular-weight PAS resin is difficult to be subjected to melt-extrusion molding, and the application thereof has been limited.
Consequently, as a method for producing such a linear, high-molecular-weight PAS resin, for example, the following method is known: A hydrous alkali metal sulfide, N-methylpyrrolidone, the amount of which is less than 1 mole per mole of the hydrous alkali metal sulfide, and a polyhaloaromatic compound are mixed, the resulting mixture is dehydrated by azeotropy to thereby obtain a composition containing a particulate anhydrous alkali metal sulfide in the form of a slurry, and the slurry is then polymerized by heating to produce a linear, high-molecular-weight PAS resin with high production efficiency (refer to, for example, PTL 1).
The aforementioned method can suppress side reactions and efficiently produce a linear, high-molecular-weight PAS resin. However, since the polymerization is conducted by a heterogeneous reaction in which the starting materials are allowed to react with each other in the form of a slurry, a slight amount of water such as crystal water remaining in the reaction system induces side reactions, and the molecular weight of the resin does not reach the recently desired high level.
In addition, the following method for producing a PAS resin has been proposed: In producing a PAS resin by allowing an alkali metal sulfide to react with a dihaloaromatic compound in an organic amide solvent by heating, polymerization is conducted by using an anhydrous alkali metal sulfide having a purity of 95% or more and containing an alkali metal hydrosulfide in an amount of 2% by mass or less, and by adding water in an amount of 0.1 to 0.8 moles per mole of the alkali metal sulfide so as to control a charging concentration of the alkali metal sulfide to be 2.5 to 5 mole/L (refer to, for example, PTL 2).
However, in the aforementioned method for producing a PAS resin using an anhydrous alkali metal sulfide, since the anhydrous alkali metal sulfide is a solid, the polymerization reaction does not proceed if no additional treatment is performed. Therefore, it is necessary to dissolve the solid anhydrous alkali metal sulfide and to supply the alkali metal sulfide as a sulfiding agent to the reaction system. In PTL 2, water is added so as to accelerate the dissolution of the solid anhydrous alkali metal sulfide. However, this water induces side reactions, resulting in a problem of difficulty in achieving a high molecular weight.